The principle objective of this research is to learn how myocardial cells react to ischemic injury and to reperfusion following periods of ischemia which cause reversible or irreversible cell injury. We have previously shown that reperfusion of canine myocardium which has been severely ischemic for 40 minutes, in vivo, is associated with explosive cell swelling, sarcolemma disruption and development of contraction bands, all of which result in myofibrillar contraction band necrosis. These changes develop within two minutes of reperfusion. Some of the projects proposed will examine the cause(s) of these reperfusion mediated events. In particular, the role of calcium overload, of reoxygenation, per se, and of superoxide production will be assessed by manipulating the constituents of the blood or electrolyte media used for reperfusion. Tissue damage will be evaluated from ultrastructural observations, from measurement of tissue electrolyte and water content, and from studies of cell function in slices incubated in vitro. In a second proposed project, we will evaluate the rate of adenine nucleotide recovery in tissue exposed to brief periods of ischemia which cause reversible cell injury. Preliminary studies have demonstrated prolonged depletion of ATP after 15 minutes of ischemia even though the cells remain viable. We plan to determine whether post-ischemic recovery of adenine nucleotides can be acclerated by intravenous infusion of ribose and/or puriune bases or nucleosides. In a third series of projects, we will develop a model of partial coronary constriction in open chest dogs with the goal of producing areas of moderate ischemia. This model will be used to characterize the sequential effects of moderate ischemia (flow = 15-30% of control) on myocardial high energy phosphate and adenine nucleotide contents, ultrastructure, and capacity to maintain cell volume regulation and electrolyte gradients. The response of myocytes to moderate ischemia will be compared with the response to severe ischemia (flow, 15% of control) with particular emphasis on the relationships between marked ATP and adenine nucleotide depletion and the onset of membrane damage and cell death. The above studies will contribute to our understanding of the pathogenesis of ischemic cell death and may have clinical applications 1) for cardiac preservation during surgery and 2) for the rational development of the means to limit myocardial infarct size.